I. Field
The following description relates generally to communications systems, and more particularly to applying frequency domain equalization in the presence of time-varying channels while mitigating effects of such channels.
II. Background
Wireless communication systems are widely deployed to provide various types of communication content such as voice, data, and so forth. These systems may be multiple-access systems capable of supporting communication with multiple users by sharing the available system resources (e.g., bandwidth and transmit power). Examples of such multiple-access systems include code division multiple access (CDMA) systems, time division multiple access (TDMA) systems, frequency division multiple access (FDMA) systems, 3GPP Long Term Evolution (LTE) systems, and orthogonal frequency division multiple access (OFDMA) systems.
Generally, a wireless multiple-access communication system can concurrently support communication for multiple wireless terminals that communicate with one or more base stations via transmissions on forward and reverse links. The forward link (or downlink) refers to the communication link from the base stations to the terminals, and the reverse link (or uplink) refers to the communication link from the terminals to the base stations. This communication link may be established via a single-in-single-out, multiple-in-single-out or a multiple-in-multiple-out (MIMO) system.
A MIMO system employs multiple (NT) transmit antennas and multiple (NR) receive antennas for data transmission. A MIMO channel formed by the NT transmit and NR receive antennas may be decomposed into NS independent channels, which are also referred to as spatial channels, where NS≦min{NT, NR}. Generally, each of the NS independent channels corresponds to a dimension. The MIMO system can provide improved performance (e.g., higher throughput and/or greater reliability) if the additional dimensionalities created by the multiple transmit and receive antennas are utilized.
A MIMO system also supports time division duplex (TDD) and frequency division duplex (FDD) systems. In a TDD system, the forward and reverse link transmissions are on the same frequency region so that the reciprocity principle allows estimation of the forward link channel from the reverse link channel. This enables an access point to extract transmit beam-forming gain on the forward link when multiple antennas are available at the access point.
Recently, frequency domain equalization (FDE) has been investigated to compensate for the impairments of multi-path channels having long impulse response. By transforming operations in the time domain into the frequency domain, FDE can reduce receiver complexity. The development of the FDE is based on the correspondence between the circular convolution of two sequences in the time domain and the element-wise product of their frequency responses. If the information data are forced to be periodic and the channel is assumed to be static during the transmission of one block of data (referred to as quasi-static channels), then the received signal's spectrum is identical to the product of the frequency response of the channel and the spectrum of transmitted data. However, the equivalence between time domain and frequency domain is no longer true if the channel is not static. The non-equivalence due to a time-varying channel degrades the performance of FDE. As a result, FDE is not effective to combat the distortion caused by time-varying channels.
In spite of the degradation in performance in time varying channels, FDEs are applied under an implicit assumption that the time-varying channel is actually static, without correcting for channel time-variation.